A plugged honeycomb structure is widely used as a dust-collecting filter for exhaust gas, for example, as a diesel particulate filter (DPF) for trapping and removing particulate matter (particulates) such as soot contained in exhaust gas from a diesel engine or the like (see, e.g., Patent Document 1).
As shown in FIG. 7, such a plugged honeycomb structure 11 used as a dust-collecting filter is provided with a honeycomb structure 2 having porous partition walls 4 separating and forming a plurality of cells 5 communicating between an inlet end face A on an inlet side of a fluid and an outlet end face B on an outlet side of the fluid, and plugging portions 3 for plugging an opening portion of each of predetermined cells 5 on the inlet end face A and an opening portion of each of the other cells 5 on the outlet end face B. Generally, adjacent cells 5 are alternately plugged in such a manner that the inlet end face A and the outlet end face B show complementary checkerwise patterns.
In the case of using a plugged honeycomb structure 11 having such a structure as a DPF or the like, when gas G1 to be treated is introduced into cells 5 from the inlet end face A, particulate matter such as soot is trapped in the partition walls 4, and treated gas G2 flowing into the adjacent cells 5 by passing through the porous partition walls 4 is discharged from the outlet end face B, thereby obtaining treated gas G2 where particulate matter in the gas G1 to be treated is separated.
By the way, a plugged honeycomb structure which has conventionally been used as a DPF or the like has a flat end face on the inlet end face A side of each of the plugging portions 3 as shown in FIG. 8. When a plugged honeycomb structure 11 is arranged in an exhaust gas flow passage, an exhaust gas flow passage area is drastically reduced on the inlet end face A of the plugged honeycomb structure 1. Then, the fluid striking the plugging portions 3 causes a stagnation of flow on the inlet end face A as shown in FIG. 8, which affects the flow of the fluid into the opening cells between the plugging portions 3 to cause a flow stagnation of the fluid flowing into the cells 5 in the vicinity of the inlet end face A and cause a peel-away vortex. As a result, pressure loss rises in the vicinity of the inlet end face A. In addition, by the excess deposition of soot on the inlet end face A due to the flow stagnation in the vicinity of the inlet end face A, the opening portions of the cells 5 are clogged with soot in a short period of time to cause deterioration in filter performance.
Patent Document 1: JP-A-2001-300922